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Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysicsM. Gatu Johnson, A. B. Zylstra, A. Bacher, C. R. Brune, D. T. Casey, C. Forrest, H. W. Herrmann, M.Hohenberger, D. B. Sayre, R. M. Bionta, J.-L. Bourgade, J. A. Caggiano, C. Cerjan, R. S. Craxton, D. Dearborn,M. Farrell, J. A. Frenje, E. M. Garcia, V. Yu. Glebov, G. Hale, E. P. Hartouni, R. Hatarik, M. Hohensee, D. M.Holunga, M. Hoppe, R. Janezic, S. F. Khan, J. D. Kilkenny, Y. H. Kim, J. P. Knauer, T. R. Kohut, B. Lahmann, O.Landoas, C. K. Li, F. J. Marshall, L. Masse, A. McEvoy, P. McKenty, D. P. McNabb, A. Nikroo, T. G. Parham, M.Paris, R. D. Petrasso, J. Pino, P. B. Radha, B. Remington, H. G. Rinderknecht, H. Robey, M. J. Rosenberg, B.Rosse, M. Rubery, T. C. Sangster, J. Sanchez, M. Schmitt, M. Schoff, F. H. Séguin, W. Seka, H. Sio, C. Stoeckl,and R. E. Tipton
Citation: Physics of Plasmas 24, 041407 (2017); doi: 10.1063/1.4979186View online: http://dx.doi.org/10.1063/1.4979186View Table of Contents: http://aip.scitation.org/toc/php/24/4Published by the American Institute of Physics
Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics
M. Gatu Johnson,1 A. B. Zylstra,2 A. Bacher,3 C. R. Brune,4 D. T. Casey,5 C. Forrest,6
H. W. Herrmann,2 M. Hohenberger,6 D. B. Sayre,5 R. M. Bionta,5 J.-L. Bourgade,7
J. A. Caggiano,5 C. Cerjan,5 R. S. Craxton,6 D. Dearborn,5 M. Farrell,8 J. A. Frenje,1
E. M. Garcia,6 V. Yu. Glebov,6 G. Hale,2 E. P. Hartouni,5 R. Hatarik,5 M. Hohensee,5
D. M. Holunga,5 M. Hoppe,8 R. Janezic,6 S. F. Khan,5 J. D. Kilkenny,8 Y. H. Kim,2
J. P. Knauer,6 T. R. Kohut,5 B. Lahmann,1 O. Landoas,7 C. K. Li,1 F. J. Marshall,6 L. Masse,5
A. McEvoy,2 P. McKenty,6 D. P. McNabb,5 A. Nikroo,5 T. G. Parham,5 M. Paris,2
R. D. Petrasso,1 J. Pino,5 P. B. Radha,6 B. Remington,5 H. G. Rinderknecht,5 H. Robey,5
M. J. Rosenberg,6 B. Rosse,7 M. Rubery,9 T. C. Sangster,6 J. Sanchez,5 M. Schmitt,2
M. Schoff,8 F. H. S�eguin,1 W. Seka,6 H. Sio,1 C. Stoeckl,6 and R. E. Tipton5
1Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA2Los Alamos National Laboratory, Los Alamos, New Mexico 87544, USA3Indiana University, Bloomington, Indiana 47405, USA4Ohio University, Athens, Ohio 45701, USA5Lawrence Livermore National Laboratory, Livermore, California 94550, USA6Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA7CEA, DAM, DIF, F-91297 Arpajon, France8General Atomics, San Diego, California 92186, USA9Plasma Physics Department, AWE plc, Reading RG7 7PR, United Kingdom
(Received 14 October 2016; accepted 12 December 2016; published online 28 March 2017)
This paper describes the development of a platform to study astrophysically relevant nuclear
reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition
Facility laser facilities, with a particular focus on optimizing the implosions to study charged-parti-
cle-producing reactions. Primary requirements on the platform are high yield, for high statistics in
the fusion product measurements, combined with low areal density, to allow the charged fusion
products to escape. This is optimally achieved with direct-drive exploding pusher implosions using
thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which
would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the
implosions is also considered. The platform is initially employed to study the complementary
T(t,2n)a, T(3He,np)a and 3He(3He,2p)a reactions. Proof-of-principle results from the first experi-
ments demonstrating the ability to accurately measure the energy and yields of charged particles
are presented. Lessons learned from these experiments will be used in studies of other reactions.
The goals are to explore thermonuclear reaction rates and fundamental nuclear physics in stellar-
like plasma environments, and to push this new frontier of nuclear astrophysics into unique regimes
not reachable through existing platforms, with thermal ion velocity distributions, plasma screening,
and low reactant energies. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4979186]
I. INTRODUCTION
Thermonuclear reactions relevant to stellar nucleosynthe-
sis (SN) and big-bang nucleosynthesis (BBN) have been
explored traditionally by means of accelerator experiments.1
High-energy-density (HED) plasmas generated in inertial con-
finement fusion (ICF) experiments at large lasers such as
OMEGA2 and the National Ignition Facility (NIF)3,4 more
closely mimic an astrophysical environment in several ways.
The target nuclei in accelerator experiments are surrounded
by bound electrons, while electrons occupy mainly continuum
states in stars and in HED plasmas. Unsatisfactory systematic
uncertainties are introduced by the two-step theoretical cor-
rections currently required to go from laboratory-measured
reaction rates with bound electron screening via projected
bare nucleus rates to stellar rates with plasma screening.1,5,6
Accelerator experiments also use a mono-energetic ion beam
to initiate reactions, while reactions in stars and HED plasmas
occur within populations of ions with thermal velocity distri-
butions. Additionally, due to the very low reaction rates at
energies relevant to SN, accurate accelerator measurements
also require very long beam times (of order months) and
extremely low background environments, such as can only be
achieved in underground facilities like the Laboratory for
Underground Nuclear Astrophysics (LUNA).7 These chal-
lenges further motivate work to develop a new method for
probing these reactions.
Astrophysical conditions span a broad range of tempera-
tures and densities. During BBN, nuclei were formed in the
time window 200–1000 s, when temperatures were in the
range of 30–80 keV (86 keV–109 K).1 The energy-producing
fusion reactions in our sun, which is currently in the main
sequence, occur at a temperature of �1.3 keV. Higher-mass
stars and later stages of stellar burning (e.g., core helium
burn, shell burn) occur at higher temperatures (Fig. 1(a)).
Conditions similar to those in stars can be closely replicated8
1070-664X/2017/24(4)/041407/17/$30.00 Published by AIP Publishing.24, 041407-1
PHYSICS OF PLASMAS 24, 041407 (2017)
in HED plasma physics experiments, where ion temperatures
(Tion) up to �30 keV (Ref. 9) and areal densities (qR) up to
�1.3 g/cm2 (Ref. 10; corresponding to densities ranging
from many tens of g/cm3 in the center of the implosion to
many hundreds of g/cm3 in the surrounding dense fuel layer)
have been demonstrated.
The use of HED plasmas to study nuclear reactions rele-
vant to SN and BBN has recently begun with implosion
experiments at the OMEGA laser11,12 and a first series of
experiments currently underway at the NIF. This work builds
on the successful use of plasmas13 and the HED plat-
form14–16 to probe fundamental nuclear physics problems.
The feasibility of accurate reaction-rate measurements using
HED plasmas by exploiting the yield ratio between the reac-
tion of interest and a well-known reference reaction has also
been recently confirmed.8 In this paper, considerations for
optimizing the HED platform for measurements of charged-
particle producing SN and BBN-relevant reactions are dis-
cussed, and results from the first platform development
experiments at both OMEGA and the NIF are presented. The
ultimate goals of these efforts are to explore thermonuclear
reaction rates and relevant fundamental nuclear physics in
stellar-like plasma environments, compare results to tradi-
tional accelerator experiments, and push the frontier into
regimes not reachable through existing platforms (with ther-
mal ion velocity distributions, plasma screening, and low
reactant energies).
Reactivities for nucleosynthesis-relevant reactions17 are
generally low, and increase very strongly with Tion (Fig.
1(b)). Given the low reactivities, the primary requirement on
a platform designed to study astrophysically relevant nuclear
reactions is high yield. Generally, the higher the yield from
an implosion, the more straightforward it will be to obtain
sufficient statistics in fusion product measurements. For
charged-particle-producing reactions, an additional neces-
sary platform requirement is minimal spectral distortions.
Escaped charged-particle spectra can be distorted either
through energy upshifts due to capsule charging, or through
energy loss in the assembled plasma.18 Energy loss scales
directly with implosion qR, which must thus be minimized
for these implosions. Charging of ICF capsules has been
found to occur because of hot electrons generated by the
two-plasmon decay (TPD) instability.19,20 Hot-electron pro-
duction due to the TPD instability scales with laser intensity
above a threshold �2–5� 1014 W/cm2 depending on plasma
conditions.20 The associated positive target potential, respon-
sible for upshifts of fusion products, decays fairly rapidly
after laser turn-off.18,19 In practice, this means that there are
two paths to minimizing charged-particle spectral distortion
due to capsule charging, (i) to keep the laser intensity below
the threshold for TPD onset, or (ii) to design the implosion
so that peak nuclear production (bang time (BT)) occurs after
the end of the laser pulse. Here, we have chosen the second
option, because reducing the laser intensity below the TPD
threshold also reduces achievable implosion yields and
makes it more challenging to achieve low qR. For many of
the reactions relevant to SN, optimizing the implosions for
low (by HED standards) Tion also increases the relevance for
stellar scenarios. In these cases, the platform design comes
down to a balance game of generating sufficient yield at low-
est possible Tion. In this paper, efforts to develop optimized
experimental designs considering these criteria to probe
charged-particle-producing reactions relevant to SN on
OMEGA and the NIF are described.
The structure of the paper is as follows: Sec. II details
design considerations for OMEGA experiments; Sec. III
results from initial OMEGA experiments. Section IV dis-
cusses design considerations for NIF experiments, and Sec.
V results from initial NIF measurements. In Sec. VI, some
aspects of data interpretation common to NIF and OMEGA
are discussed; in particular, how does the fact that these ICF
implosions are not uniform in density and temperature over
the burn duration and region impact how we interpret the
data? Finally, future directions and planned platform
improvements are discussed in Sec. VII, and Section VIII
concludes the paper.
FIG. 1. (a) Typical density and temperature conditions under which fusion reactions occur in stars with mass equivalent to 1 solar mass (1 MSun, green), 10
MSun (blue), and 40 MSun (red) evolve over time (from left to right in the figure) as the star passes through the main sequence, core helium burn and shell
burn. Note that while time moves from left to right in the figure, the absolute time scale is different for each trace—lighter stars evolve much slower than
heavier stars. (b) Reactivities for SN and BBN-relevant reactions are generally low and decrease rapidly with decreasing temperature. Here, as an example,
BBN-relevant reactions T(3He,d)a and 3He(a,c)7Be are shown in green, proton-proton-chain-relevant reactions D(p,c)3He and 3He(3He,2p)a in black, CNO-
relevant reactions 15N(p,a)12C and 14N(p,c)15O in blue, and reference reactions D(T,n)a, D(D,n)3He, D(3He,p)a, and T(T,2n)a in red.
041407-2 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
II. OMEGA EXPERIMENT DESIGN
The dual platform requirements of high yield and mini-
mal qR are optimally met with shock-driven, thin-glass-shell
capsule “exploding pusher” implosions. Design optimization
for the first round of platform development experiments on
OMEGA focused on selecting the optimal target parame-
ters—outer diameter (OD), shell thickness, and fill pressure
(P)—to maximize yield and minimize qR at maximum laser
power. Optimal laser pulse duration was also considered.
These first implosions were designed to measure the neutron
and alpha fusion product spectra from the tritium-tritium
T(t,2n)a reaction, a mirror reaction to 3He(3He,2p)a, which is
responsible for nearly half the energy production in our sun.1
1D ARES21-simulated TT neutron (TT-n) yield, burn-
averaged Tion and qR for two T2 fill pressures (P¼ 5 atm and
P¼ 10 atm), and three ODs (OD¼ 860 lm, 920 lm, and
1160 lm) are shown versus shell thickness in Fig. 2. These
simulations were all run with a 1 ns square laser pulse with
realistic wiggles and laser energy Elaser � 28 kJ. 75% of the
laser energy was assumed to be absorbed by the capsule, and
a flux limiter of 0.07 was used. Reactions from zones with
Tion> 25 keV were excluded in the post-processing to mini-
mize spurious values from central zones, where a numerical
singularity in 1D creates unphysical temperatures. The model
also included 0.25 atm residual CO2. The first important
observation is that qR varies relatively slowly, and is accept-
ably low, for shell thickness �3 lm. Yield is seen to climb
over the range of shell thicknesses plotted for all but the larg-
est capsules (OD¼ 1160 lm), which show a pressure-
dependent roll-over (the yield starts to fall earlier with shell
thickness for the 10 atm fill case than for the 5 atm fill).
Based on these simulations, 1000 lm OD capsules with
3.0 lm-thick shells and �10 atm T2 fill were selected for the
first round of T2 implosions to maximize yield at an accept-
able (�5 mg/cm2) qR. The database of past DT-gas-filled
thin-glass-shell OMEGA implosions (2004–2010) was also
studied to confirm the fidelity of the simulation results
(Fig. 3). Not only shell thickness but also capsule diameter,
fill pressure, and laser energy vary between different implo-
sions, which has to be considered when studying Fig. 3. The
available data suggest that a DT yield of �6� 1013 is
obtained around a shell thickness of 3.0 lm and
OD¼ 1000 lm for a fill pressure of �10 atm for these 50:50
D:T implosions, which scales to a TT-n yield of �1012,
roughly consistent with the simulated result.
Simulations and past data also show that if a 1000 lm
OD capsule with a 3.0 lm-thick shell and �10 atm fill is shot
with a 1 ns square, full power laser pulse, bang time (BT)
will occur during the laser pulse. As discussed above, this
can be expected to lead to charged-particle energy upshift
due to capsule charging.18 This motivated running a simula-
tion with a 0.6 ns laser pulse at full power for comparison.
The ARES simulations predict a minimal TT-n yield reduc-
tion of 4.9% by going from the 1.0 ns to the 0.6 ns pulse. DT
bang times of 864 ps for the 0.6 ns case and 896 ps for the
1.0 ns case are predicted, which indicates that at a minimal
cost in yield, the outlook for undistorted charged-particle
measurements is much improved.
So far, we have not discussed Tion at all. As can be seen in
Fig. 2, Tion is predicted to stay fairly constant as a function of
shell thickness for the nominal drive conditions for thicknesses
�3.0 lm, and to drop towards higher shell thicknesses
(although this would come at the expense of higher qR).
Predicted Tion for these implosions is generally fairly high; opti-
mizing for low Tion was not a primary goal for the first
OMEGA experiments. It is interesting to note, however, that if
we relax the qR requirements, Tion can be significantly varied
using the same glass capsules by changing the fill pressure/laser
pulse combination. This was tested in the first round of platform
development experiments at OMEGA (see Section III below).
III. OMEGA EXPERIMENT RESULTS
Table I summarizes implosion parameters and results
for the first platform development T(t, 2n)a experiments
on OMEGA November 2012-January 2013 (shots
67 941–68 448). These implosions all used SG4 phase plates,
but smoothing by spectral dispersion (SSD) was not applied.
0
5
10
15
20
2 2.5 3 3.5 4 4.5
Tota
l ρR
(mg/
cm2 )
Shell thickness (µm)
0
5
10
15
20
Bur
n-av
erag
ed T
ion
(keV
)
0.0
5.0
1.0
1.5
2.0
2.5
3.0
3.5
1D C
lean
TT-
n yi
eld
x1012
(a)
(b)
(c)
FIG. 2. (a) TT-n yield, (b) burn-averaged Tion, and (c) total qR from 1D
ARES simulations of OMEGA-scale implosions versus SiO2-capsule shell
thickness (see text for details). Blue stars represent capsules with 5 atm T2
fill and 860 lm OD, blue diamonds capsules with 10 atm T2 fill and
860 lm OD, red squares capsules with 10 atm T2 fill and 920 lm OD,
green triangles capsules with 5 atm T2 fill and 1160 lm OD, and purple
crosses capsules with 10 atm T2 fill and 1160 lm OD. Note in particular
how simulated total qR (bottom panel) starts to climb rapidly for shell
thickness > 3 lm.
041407-3 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
For comparison, results from similar experiments in July 2015
(shots 77 951–77 964) are also shown. Two reconfigurations of
the OMEGA facility happened between the two series of
shots—new, smaller SG5 phase plates were introduced, and an
arbitrary waveform generator (AWG) was implemented (this
is why the pulse shapes change names between the two sets
of shots; nominally, SG0604¼SG06v001 and RM2002
¼RM20v001). The TT-n yields were measured with a highly
collimated, well-shielded neutron time-of-flight (nTOF) detector
at 13.4 m from the target chamber center (TCC) with the capa-
bility of gating out the faster neutrons from the DT reaction;22
for shots 67 952–67 963, a bibenzyl scintillation crystal was
used, while shots 67 941, 68 448, and 77 951–77 964 used an
oxygenated xylene scintillator. (On shots 67 952–67 963, inde-
pendent TT-n yield measurements were also made with the
magnetic recoil spectrometer (MRS) neutron spectrometer and
with indium, aluminum, and copper activation; good agreement
was found between the three methods, see Appendix A.) DT
Tion for shots 67 941–68 448 was measured with the 12mntofh
detector,23 DT Tion for 77 951–77 964 with the 13.4 m LaCave
nTOF detector. Bang times were measured with the neutron
temporal diagnostic (NTD)24 (the uncertainty in the measured
DT BT is�650 ps).
The first thing to note when studying Table I is that the
performance for nominally identical implosions is remark-
ably stable over time, in spite of the facility changes
TABLE I. Parameters of 13 tritium-gas filled, glass-shell implosions run in the first series of OMEGA experiments using the platform described in this paper
(November 2012-January 2013; shots 67 941-68 448), and of an additional 5 comparable implosions from July 2015 (shots 77 951-77 964). Laser pulse shapes
SG0604 and SG06v001 are nominally the same, 0.6 ns square, SG1018 is 1.0 ns square, and RM2002 and RM20v001 are nominally the same, 2 ns ramped (see
Fig. 4).
Shot
Pulse
shape
Laser
energy (kJ)
Lens
position (mm)
Capsule
diameter (lm)
Shell
thickness (lm)
T2 fill pressure
(atm)
TT-n yield
(�1012)
DT Tion
(keV)
DT bang
time (ps)
67 952 SG0604 17.5 0 1006 2.9 10.0 6 0.2 1.40 6 0.16 13.2 6 0.5 843
67 953 SG0604 17.7 0 1007 2.9 10.0 6 0.2 1.40 6 0.16 12.8 6 0.5 867
67 954 SG0604 17.8 0 1008 2.9 10.0 6 0.2 1.16 6 0.14 13.8 6 0.5 888
67 955 SG1018 29.7 0 1012 2.9 9.9 6 0.2 1.24 6 0.14 13.8 6 0.5 878
67 956 SG1018 29.2 0 1012 2.9 10.0 6 0.2 1.35 6 0.16 13.2 6 0.5 917
67 958 SG1018 28.8 0 1013 2.9 10.0 6 0.2 1.21 6 0.14 12.8 6 0.5 905
67 959 SG1018 29.3 0 1017 3.0 10.0 6 0.2 1.42 6 0.17 13.1 6 0.5 911
67 960 RM2002 22.4 0 1011 3.0 10.0 6 0.2 0.59 6 0.07 7.5 6 0.5 …
67 961 RM2002 22.4 0 1028 3.3 9.9 6 0.2 0.55 6 0.06 6.7 6 0.5 1619
67 962 RM2002 22.8 0 1021 3.3 10.0 6 0.2 0.55 6 0.06 6.5 6 0.5 1594
67 963 RM2002 23.2 0 1011 3.3 9.9 6 0.2 0.60 6 0.07 6.6 6 0.5 1516
67 941 RM2002 23.6 7.3 1027 3.3 9.9 6 0.2 0.18 6 0.02 4.1 6 0.5 …
68 448 RM2002 21.1 7.3 1007 2.9 9.9 6 0.3 0.13 6 0.01 4.0 6 0.5 …
77 951 SG06v001 16.1 0 1004 2.9 3.3 0.49 6 0.05 18.3 6 0.5 756
77 952 SG06v001 15.9 0 1005 2.9 3.3 0.40 6 0.04 17.7 6 0.5 731a
77 960 SG06v001 16.1 0 1004 2.9 8.2 1.27 6 0.14 11.1 6 0.5 806a
77 963 RM20v001 24.1 7.3 1009 3.0 8.2 0.24 6 0.03 3.7 6 0.5 1689a
77 964 RM20v001 19.3 7.3 1007 2.9 8.2 0.13 6 0.01 3.4 6 0.5 1763a
aData from the new cryoNTD detector; all other bang times measured with the old H5 NTD.
FIG. 3. Data from OMEGA DT exploding pusher implosions from 2004 to 2010 showing DT yield dependence on (a) SiO2 shell thickness, (b) capsule diame-
ter, and (c) DT fill pressure. Blue diamonds represent implosions with 20 atm DT fill shot with 28–30 kJ laser energy on target, red squares implosions with
10 atm DT fill shot with 20–25 kJ laser energy, gray triangles capsules with 5 atm DT fill shot with 25–28 kJ laser energy, and green circles implosions with
2.4–4.4 lm thick SiO2 shells shot with 20–30 kJ laser energy. The spread in the plotted data is expected as shell thickness, capsule diameter, fill pressure, and
laser energy each individually impact yield performance and more than one such implosion parameter varies within each set of data points.
041407-4 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
mentioned and in spite of the diagnostic changes between
implosions (compare shots 67 952–67 954 and shot 77 960,
and shots 67 941, 68 448, 77 963, and 77 964). The second
thing to note is that the agreement with 1D simulations is
remarkably good. The 1D ARES-predicted TT-n yield for
the 1-ns square laser pulse implosions is �1.4� 1012 (Fig.
2(a)), while the measured average for these implosions is
1.3� 1012 with an uncertainty (dominated by systematics) of
�11%. ARES predicts a 4.9% increase in TT-n yield going
from a 0.6 ns to a 1.0 ns square pulse; on average, a 1.1%
reduction in TT yield is observed. For comparison, ARES
predicts a 3.7% increase in DT yield, while on average, an
increase of 14% is observed. (DT yields are not shown in the
table because the deuterium impurity (�1.5%) in the T2 fill
is not well known, see Appendix B.) This confirms the con-
clusion from simulations that only minimal (if any) yield
reduction is expected when going from a 1.0 ns to a 0.6 ns
square pulse for a capsule of these dimensions. Further,
ARES also predicts a DT BT of 864 ps for the 0.6 ns case
and a DT BT of 896 ps for the 1.0 ns case; average DT BTs
of 866 ps and 891 ps are observed, again in remarkable
agreement with the (pre-shot) 1D simulation.
Backscattered laser light was also measured on these
experiments.25 From these measurements, an absorbed laser
light fraction of �66% was observed for the high-intensity
0.6-ns and 1.0-ns square laser pulses, while for the lower
intensity RM2002 laser pulse shots, �75% of the laser light
was absorbed by the capsule. When comparing the simula-
tions and measurements, it should be kept in mind that a
slightly higher capsule absorption (75%) was assumed in the
simulations.
In addition to the 0.6 ns and 1.0 ns square pulse cases, a
2 ns ramped laser pulse was also shot with two different
focusing settings (best-focus, with the lens position set to
0 mm, and de-focus, with the lens position set to 7.3 mm; the
lens is a component of the final optics for each laser beam
that determines the beam focus26). In July 2015, a lower fill
pressure (PT2 � 3.3 atm) was also shot. The purpose of these
variations was to vary Tion to study the energy dependence of
the T(t, 2n)a reaction. As can be seen in Table I, Tion ranging
from 3.4 keV to 18.3 keV was achieved this way. The highest
Tion of 18.3 keV is obtained by driving the capsule with the
lower fill pressure with a square pulse, leading to a fast,
entirely shock-driven implosion. The lowest Tion of 3.4 keV
is obtained by imploding a higher fill-pressure capsule with
the lower-intensity 2 ns ramped pulse and defocusing the
laser beams, thus driving the implosion slower and more
compressively.
Fig. 4 shows the laser pulse shapes and NTD-measured
burn histories from shots 67 953, 67 956, and 67 961. The
laser pulse shapes are representative for the 0.6 ns square,
1.0 ns square, and 2 ns ramped cases, respectively. Note that
the DT neutron burn histories for the 0.6 ns square (black)
and 1.0 ns square (red) cases are very similar, with burn hap-
pening well within the laser pulse for the 1.0 ns square case
and after the end of the laser pulse for the 0.6 ns case. For
these two implosion types, the NTD was fielded 20 cm from
TCC to avoid saturation because of the relatively high yield.
For the 2 ns ramped case (shot 67 961, blue), NTD could be
moved to 10 cm from TCC, which reduced the time
separation between the DT and TT neutrons. The closer fiel-
ding distance also reduces the time spread for the TT neutron
spectrum, which extends over energies �0–9 MeV, making
the TT signal stronger relative to DT. In this case, the TT neu-
trons can also be seen in the trace (from �2000 to 3000 ps;
note that the time window covered by the NTD streak camera
sweep time also changes with the fielding distance, which
contributes to the TT signal being visible for 67 961 and not
for 67 953 and 67 956). Bang time happens well within the
laser pulse also for the 2.0 ns ramped case, but we will see
below that the charged-particle spectral distortions are still
dominated by downshift due to qR because of the higher con-
vergence obtained when the implosion is driven this way.
The success in optimizing these implosions for minimal
distortion in charged particle measurements can be evaluated
by studying the alpha particle spectra measured by two
charged-particle spectrometers27 (CPS1 & CPS2). An exam-
ple alpha spectrum measured by CPS2 for shot 67 952 is
shown in Fig. 5. The DT alphas (Ea � 3.6 MeV) clearly domi-
nate the measurement. The TT-alpha spectrum is expected to
cover the range from 0–4 MeV, but cannot be reliably mea-
sured in these implosions with �1.5% deuterium because of
the high DT yield. A hint of TT alphas can be seen in the
range 1.5–2.0 MeV, but the shape of the TT alpha spectrum
cannot be trusted because of possible contamination from
scattered DT-alpha and challenges in the subtraction of a
background of accelerated Si and O from the capsule shell
towards the low end of this energy range. Also shown in the
plot is the expected DT alpha spectrum, broadened and
upshifted28 consistent with Tion inferred from DT neutron
measurements on this shot (13.2 keV; red curve). The mea-
sured spectrum appears slightly downshifted and slightly
broadened relative to the expectation (a Tion¼ 18.8 keV is
inferred from the DT-a spectrum, neglecting additional broad-
ening due to qR evolution or the decaying target potential18),
but given the relatively high stopping power for alpha par-
ticles,29 the agreement for this 0.6 ns pulse shape shot is
remarkably good, demonstrating that the goal of designing an
implosion where charged-particle spectra escape undistorted
was met.
FIG. 4. Laser pulse shapes (dashed lines, left axis) and NTD data (solid
lines, right axis, arbitrary units) from three example shots, 67 953 shot with
the SG0604 0.6 ns square pulse shape in black, 67 956 shot with the SG1018
1.0 ns square pulse shape in red, and 67 961 shot with the RM2002 2 ns
ramped laser pulse in blue, all with best focus (0 mm lens position). Note
that burn happens after the end of the laser pulse only for the 0.6 ns pulse
case (black).
041407-5 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
As expected, the 1.0 ns square pulse shape leads to
upshifted DT-a spectra due to capsule charging, and the 2 ns
ramped pulse shape gives downshifted DT-a spectra due to
higher total implosion qR (Fig. 6). The mean DT alpha energy
(Ea) for the 0.6 ns square pulse shape case comes in very close
to nominal, with a downshift consistent with qR �1 mg/cm2
(qR¼ 0.4 mg/cm2 if we assume18 Te¼ 1 keV, ni¼ 1022/cm3
and stopping in SiO2 only for the most ranged down out of
the three shots of this type, with Ea¼ 3.45 MeV). The optimal
pulse shape for charged-particle measurements is clearly the
0.6 ns square.
Another interesting question to consider in this context
is the symmetry of the charged particle emission. It has
been demonstrated30 that variations in charged-particle
yields measured in different locations around the target
chamber are also reduced when the bang time is after the
end of the laser pulse. Since alpha particle yields can only
be measured in two locations on OMEGA, T3He shot 73598
with similar implosion parameters as for the TT shots dis-
cussed above (OD¼ 981 lm, Dt¼ 3.0 lm, 16.6 kJ laser
energy, SG0604, PT2¼ 3.5 atm, P3He¼ 7 atm, D impurity
�2%) was chosen for this demonstration. D3He-proton
yields from this shot measured in five different locations
around the target chamber using the magnetic recoil spec-
trometer31 (MRS), CPS and wedge range filter32 (WRF)
proton spectrometers are plotted in Fig. 7. Yield variations
both globally and locally are seen to be insignificant within
error bars. The standard deviation of all detectors is �10%,
while a weighted average yield of 3.3� 109 6 3.2% (Ref.
33) is inferred from the ensemble of measurements.
The indication from all measurements described above
is that these implosions are reasonably 1D in nature. This is
further supported by time-integrated x-ray images of these
implosions, which indicate that the implosions are highly
spherical.
We conclude that an OMEGA platform to measure
charged-particle yields and spectra from reactions relevant to
SN and BBN has been successfully developed. The platform
has already been exploited to measure the rate of the T3He-creaction, ruling this reaction out as an explanation for the 6Li
abundance problem;11 to measure an energy-dependence of
the T(t,2n)a reaction at center-of-mass energies from 16 to
50 keV;34 and for initial studies of the proton spectra from
the 3He(3He,2p)a and T(3He,np)a reactions.12
IV. NIF EXPERIMENT DESIGN
The laser energy and power available on NIF (max
1.8 MJ, 500 TW) are much higher than at OMEGA (max
30 kJ, 30 TW). This allows NIF to generate larger plasma vol-
umes compared with OMEGA, which enables experiments
with equivalent yield, and thus similar data quality, at lower
temperature and, hence, conditions more directly relevant to
FIG. 5. Example alpha spectrum measured by CPS2 on shot 67 952 (black
points with error bars). Also shown is the expected nominal shape for the
DT alpha spectrum for this shot (broadened and upshifted consistent with
Tion¼ 13.2 keV, inferred from the DT-n spectrum; red curve). A hint of the
TT alphas can be seen in the energy range 1.5–2.5 MeV, but the DT alphas
dominate the spectrum as expected for this shot with �1.5% residual deute-
rium impurity in the fuel. Instrument design prevents detection in the gap in
the energy range 2.5–2.8 MeV.
FIG. 6. Measured mean DT alpha energy relative to nominal for the differ-
ent pulse shape cases (the nominal energy is corrected for upshifts due to
finite Tion as in Ref. 28). Hollow diamonds represent CPS2 data, solid
squares CPS1. Blue symbols represent shots where the SG1018 laser pulse
was used (bang time during the laser pulse), red symbols shots where the
SG0604 laser pulse was used (bang time after the laser pulse), and black
symbols shots where the RM2002 laser pulse was used (bang time during
the laser pulse; higher areal density).
FIG. 7. D3He proton yields measured in nine locations around the OMEGA
target chamber for 50:50 T3He shot 73598 (with �2% D impurity in the fuel).
Blue diamonds represent WRF data, the red square MRS data, the green trian-
gle CPS1, and the green circle CPS2. The two WRF modules in the P11 port
are only about 2 cm apart, and the four modules in TIM3 are within �10 cm2.
CPS1, CPS2, MRS, P11, and TIM3 are distributed around the chamber at
polar, azimuthal angles of h,u¼ 63�,198�, h,u¼ 37�,18�, h,u¼ 119�,308�,h,u¼ 117�,234�, and h,u¼ 143�,342�, respectively. Yield variations both
globally and locally are seen to be insignificant within error bars.
041407-6 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
SN. As an aside, note that much higher densities can also be
achieved at NIF than at OMEGA, as evidenced by the record
areal densities obtained at the two facilities of �1.3 g/cm2 at
NIF10 and �0.3 g/cm2 at OMEGA.35 This higher density
presents a significant advantage when attempting to obtain
high yield at low Tion when studying neutron- or gamma-
producing SN-relevant reactions the probing of which does
not require low qR. However, since the focus of this paper is
on measurements of charged-particle producing reactions, this
will not be further discussed here.
A glass-shell “exploding pusher” platform for implosions
on the NIF was initially developed to calibrate the NIF nuclear
diagnostic suite.36–40 D2, D2/3He and DT-gas-filled capsules
were imploded using this platform from 2010 through 2014,
with the primary goal of generating a large amount of neu-
trons and protons at low areal density for diagnostic calibra-
tion purposes. With varying drive conditions, these early
implosions (OD � 1600 lm, Dt� 4.5 lm) achieved DD Tion
ranging from 4.0 keV (for NIF shot N130129) to 11.3 keV (for
NIF shot N120328), DD-neutron yields up to �1012 and
acceptably low total qR’s (�10 mg/cm2) in some cases, with
laser energies varying from 43–131 kJ.38 Based on reactivity
scaling, i.e., using the reactivities from Fig. 1(b) and assuming
the same burn conditions (density and Tion) as for these refer-
ence implosions but pure 3He fuel instead of D2, a3He(3He,2p)a proton yield of �3� 107 should be achievable
at Tion¼ 11 keV. A yield of �5� 106 is estimated to be
required for a strong spectral measurement of this reaction on
the NIF. Given this, a set of three implosions with 3He, T2 and
50:50 T2/3He fills at the same initial density (�1.4 mg/cm3)
was designed, based on reference shot N120328, as a first
platform development experiment to make SN-relevant
measurements on NIF. Note that these capsules have OD
�1600lm, which nominally gives only �4� increase in burn
volume compared to the OD � 1000 lm capsules used on
OMEGA, but these experiments use less than 1/10th of the
total laser energy available on NIF and it should be possible to
push to much larger capsules in future experiments.
NIF presents the additional challenge over OMEGA that
the beam configuration is optimized for indirect drive, with
the 192 available laser beams divided in four upper and lower
cones at 23.5�, 30�, 44.5�, and 50� angles to the polar axis,
respectively. Hence, the targets have to be driven with polar-
direct drive (PDD),41 which makes it challenging to achieve a
symmetric implosion. We attempt to accomplish this by
designing a scheme for re-pointing the beams around the tar-
get for uniform illumination using the SAGE code42 and the
method described in Ref. 26. This has been shown to work
well in PDD experiments with thicker CH shells,43,44 and has
also been used to design pointings for earlier NIF PDD
exploding pusher implosions, including N120328.37 Time-
resolved x-ray images obtained on reference shot N120328
showed a fairly substantial non-uniformity with the implosion
being oblate (diameter �940 lm� 800 lm) in-flight at
t¼ 1.14 ns, even though SAGE simulations indicated that the
implosion should be symmetric. For this reason, a new point-
ing design was developed for these implosions that according
to SAGE would over-drive the capsule on the equator, com-
pensating for the observed asymmetry on N120328 (Fig. 8).
In this design, all beams were at best focus, and all beams had
the same energy. All 50� beams and the lower ring of 44.5�
beams were pointed close to the equator. Within each set of
four grouped beams (quad), two beams were shifted to the left
and two to the right for improved azimuthal uniformity.
V. NIF EXPERIMENT RESULTS
Table II summarizes implosion parameters for the first
set of NIF nuclear astrophysics platform development shots
as well as for reference shot N120328. The goals of these
shots were to study and compare the broad energy spectra of
particles from the complementary six-nucleon-systems
T(t,2n)a, 3He(3He,2p)a, and T(3He,np)a, and to compare the
many measurable nuclear yields from these three implo-
sions12 to assess the achievable accuracy in 3He3He stellar
rate measurements using this platform. The energy spectra
from these few-body reactions are interesting from a funda-
mental nuclear physics point of view; the mechanisms gov-
erning these reactions are not well understood. The spectral
data will be used to benchmark theory for the spectral shape
of the 3-body final-state spectra, both R-matrix16,45,46 and
ab-initio.47,48
Note that the implosion parameters within the set of new
shots are very similar (except for the variations in fill).
Compared to the reference shot, there are a few important
differences: (i) the laser energy delivered is higher on the
reference shot by �15% (although the same 125 kJ laser
energy was requested on all four shots), (ii) the capsule wall
is thinner on the reference shot by �0.3 lm, and (iii) the fill
pressure is higher by nearly 2� (although the density is not
that different, because of the lower deuterium mass).
The ramped laser pulse shape used on these implosions
(including on N120328) is shown in Fig. 9 together with the
x-ray burn history as measured by SPIDER49 and the DT-n
bang time as measured by MagPTOF49 for shot N160530–001.
The x-ray emission is filtered through 10.58 lm Ge (h�� 4 keV); the narrow peak with FWHM � 200 ps is expected
to correspond to the core burn, while the broader feature below
FIG. 8. (a) SAGE density contour plot from a run with the beam pointing
that was selected for the first round of NIF platform development shots. The
orange lines represent the critical density nc, and the contours outside the
outer orange line nc/2, nc/4, and nc/8, respectively. Also shown is a subset of
rays from a 50� beam. Note that the beam is repointed to below the equator
of the target. (b) A comparison between the center-of-mass radius at 1.6 ns
(in-flight) as a function of polar angle h for two different runs. SAGE pre-
dicted that the best symmetry would be obtained with the pointing design
shown in red (dashed curve), but based on earlier experience, the pointing
design shown in blue (solid curve) with a predicted over-drive on the equa-
tor was selected for the experiment.
041407-7 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
(FWHM � 800 ps) is expected to be due to SiO2 x-ray emis-
sion. Note that bang times are observed (marginally) after the
end of the laser pulse for these implosions.
Tables III–V summarize predicted and measured yields
and Tion for the first set of three NIF shots. Only yields from
reactions between the primary fill species are included in the
tables; in addition, a DT-n yield is measured for the T2-filled
implosion, a D3He-p yield for the 3He-filled implosion, and
DT-n and D3He-p yields for the T2/3He-filled implosion, but
these cannot be directly compared to predictions at this point
because the deuterium content is not well known (see
Appendix B). The TT-n yield and burn-averaged DT Tion for
the T2 and T2/3He implosions are measured using nTOF
detectors 18–22 m from the implosion.50,51 The T3He-deu-
teron yield is measured using the MRS52 and step-range-fil-
ter (SRF) detectors.53 For the 3He-filled implosion, the3He3He yield and the D3He-proton spectrum are measured
using WRF proton spectrometers54 fielded 10 cm from TCC
(Tion for this shot is estimated from the width of the D3He-p
peak). The 1D free-fall yield is inferred from a free-fall anal-
ysis55 of pre-shot 1D ARES simulations (the free-fall analy-
sis is intended to compensate for 2D effects leading to a
decrease in yield relative to 1D for these PDD implosions).
The scaled yield comes from reactivity scaling from refer-
ence shot N120328 corrected for 1D-LILAC56-simulated
expected performance increases when going from a 9.9 atm
to a 5.2 atm fill and from a 4.4 lm to 4.7 lm thick shell
(according to the 1D simulations, these two changes should
boost the yield by �25% and �5%, respectively).
It is clear from studying Tables III–V that the yields for
these implosions came in lower than predicted. This can
largely be explained by the lower-than-expected Tion. The
reactivity for the TT reaction is about 2.2 times higher at
11 keV than at 8.2 keV (Fig. 1(b)), which means that the
entire difference between the TT-n yield predicted based on
reactivity scaling from D2 reference shot N120328 and the
TT-n yield measured on N160530-001 can be explained by
the Tion difference between the two shots. Similarly, the
reactivity for the 3He3He reaction is about 155 times higher
at 11 keV than at the very roughly estimated Tion for shot
N160601-001 of 6 keV. This would also be more than
enough to explain the difference between the predicted and
measured 3He3He-p yields. However, the TT reactivity ratio
TABLE II. Parameters of the 3 first NIF platform development shots with the goal of studying reactions relevant to stellar nucleosynthesis. Values for refer-
ence shot N12032838 are also shown. (Nk is the Knudsen number, which is discussed in more detail in Section VII).
Shot
Pulse length
(ns)
Laser energy
(kJ)
Capsule diameter
(lm)
Shell thickness
(lm)
T2 fill pressure
(atm)
3He fill pressure
(atm)
Initial density
(mg/cm3)
x-ray bang
timea (ns) Nk
N160530-001 2.1 113.0 1578 4.7 5.2 … 1.30 2.05 6 0.03 0.3
N160601-001 2.1 113.6 1579 4.7 … 11.04 1.39 2.02 6 0.03 0.01
N160601-002 2.1 111.3 1594 4.6 2.65 5.98 1.42 2.07 6 0.03 0.05
N120328 (ref) 2.1 130.6 1555 4.4 9.9 atm D2 1.66 1.77 0.3
aMeasured using the SPIDER73 x-ray detector.
FIG. 9. Delivered laser power as a function of time for NIF shot N160530-
001 (dashed black curve), shown together with SPIDER-measured x-ray
emission history (solid black curve, a.u.). An x-ray bang-time of 2.05 6 0.03
is inferred from the SPIDER data from this shot. Also shown is the DT-n
bang-time inferred from the MagPTOF detector (solid red line), with the
dashed red lines representing the uncertainty in this measurement.
TABLE III. Measured and predicted TT-n yield and DT Tion for T2-only
NIF shot N160530-001. DT Tion is inferred from the width of the DT-n spec-
trum and can be measured because of a small deuterium impurity in the T2
fill (�0.15% by atom). The 1D free-fall yield comes from a free-fall analysis
of pre-shot 1D ARES simulations. The scaled yield is based on reactivity
scaling from reference shot N120328.
N160530-001 TT-n yield DT Tion (keV)
1D free-fall 1.1 � 1013
Scaled 1.7 � 1012 11
Measured (8.2 6 1.7) � 1011 8.2 6 0.2
TABLE V. Measured and predicted TT-n and T3He-deuteron yields and DT
Tion for mixed T2/3He-fill NIF shot N160601-002. The same comments as
for Table III apply.
N160601-002 TT-n yield T3He-d yield DT Tion (keV)
1D free-fall 1.5 � 1012 8 � 1010
Scaled 4.2 � 1011 6.7 � 109 11
Measured (6.9 6 1.4) � 1010 (4.5 6 0.1) � 108 7.1 6 0.2
TABLE IV. Measured and predicted 3He3He-proton yield and estimated
Tion (inferred from the width of measured D3He-proton spectra) for 3He-fill
NIF shot N160601-001. The same comments as for Table III apply.
N160601-001 3He3He-p yield Tion (keV)
1D free-fall 2.8 � 109
Scaled 3.8 � 107 11
Measured �4.1 � 105 �6
041407-8 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
going from 11 keV to 7.1 keV of 3.4 is not enough to explain
the difference between the predicted and observed TT-n
yields for shot N160601-002 (a factor 6.1). The T3He reac-
tivity ratio going from 11 keV to 7.1 keV of 7.9 is also not
enough to explain the difference between predicted and
observed T3He-d yields (a factor 15). Some additional factor
appears to be degrading the performance of the T2/3He-
mixed-fill shot relative to the reference shot.
When using the measured Tion to scale yields using the
reactivities from Fig. 1(b), it is important to keep in mind
that Tion as inferred from the neutron spectrum28 may also be
affected by flows in the implosion57–59 (this is further dis-
cussed in Section VI below). In this context, it is interesting
to compare Tion as measured by four different nTOF detec-
tors in different locations around the NIF target chamber
(Fig. 10). For the T2/3He-mix shot, DT Tion measured in
three different lines of sight distributed from 8� to 116� polar
angle is seen to agree very well with each other (v2red¼ 0.1).
For the T2-only shot on the other hand, DT Tion measured in
four different lines of sight distributed from 8� to 161� polar
angle shows signs of anisotropy (v2red¼ 1.8), with higher
Tion being inferred near the equator, which may indicate flow
broadening of the DT-n peak.
Time-resolved self-emission x-ray imaging60 was used to
diagnose symmetry for these implosions. Fig. 11 shows an in-
flight (t� 1.75 ns) x-ray image from an equatorial view (polar,
azimuthal angles h,u¼ 90�,78�) for shot N160530-001. The
implosion is strongly oblate at this time, with an equator-to-
pole aspect ratio of �1.5 (diameter �610� 400 lm), indicat-
ing that the laser drive is significantly stronger on the pole
than on the equator and that more work is required to optimize
the laser drive uniformity for these implosions. The asymme-
try persists through burn, as confirmed by self-emission
images taken around bang time for all three implosions (Fig.
12). Note that while the equatorial images (upper row in Fig.
12) consistently appear very oblate, the polar images (lower
row in Fig. 12) are nearly round. This is consistent with the
beam symmetry on NIF. A burn volume of 5� 10�3 mm3 is
estimated from the N160601-001 data.
An all-important question in terms of the goals of these
shots concerns the symmetry and level of distortion of
charged-particle fusion products emitted from the implosions.
Fig. 13 shows the D3He-p yields measured in different loca-
tions around the target chamber on T2/3He-mix shot
N160601-002, using the MRS, WRF, and SRF detectors.
MRS is located at polar, azimuthal angles h,u¼ 73�,324� and
the WRF and SRF detectors are held by three diagnostic-
insertion modules (DIM) at h,u¼ 0�,0�, h,u¼ 90�,78� and
h,u¼ 90�,315�, respectively. (The SRF and WRF modules
connect to each DIM at 613� and 63.5� from the DIM axis.)
Note that while the MRS and WRF detectors give an average
yield value in their respective lines of sight, the SRFs are flat-
filter detectors each covering a 5 cm-diameter round area at
�50 cm from TCC (�0.01 sr), and thus allow study of local
variations in D3He-p fluence. The black error bars in Fig. 13
span the range from maximum to minimum yield inferred
across a single SRF module. This shows that local yield varia-
tions are as large as or larger than global yield variations
observed between the MRS and the three DIMs. The standard
deviation of all measurements is �12%, which is a bit high
for an implosion with bang-time after the end of the laser
pulse.30 Using all 12 samples, the total D3He-p yield from the
implosion is constrained to 2.7� 108 6 8.5%.61 This total
uncertainty is higher than observed on a similar, smaller scale
implosion on OMEGA (Fig. 7). Anisotropies in fluence
around the implosion result from deflections in electromag-
netic fields, the topology of which can be expected to be
impacted by the symmetry of the implosion. The current
hypothesis is that reduced asymmetry in the implosion will
also reduce yield fluctuations.
In Fig. 14, D3He-proton energy spectra measured in dif-
ferent locations around the implosion are compared to the
nominal expected D3He-p spectrum (dashed line, calculated
assuming Tion¼ 7.1 keV, YD3Hep¼ 2.7� 108, and an instru-
ment resolution of 159 keV). The average spectrum mea-
sured by the two WRFs on the pole (red curve) compares
well with the nominal spectrum, with only minor distortion
and a �0.15 MeV downshift, indicating little qR on the pole.
Spectra measured closer to the equator (MRS in gray, aver-
age of equatorial WRFs in black) show substantially more
distortion with a significant low-energy tail, and are also
noticeably more downshifted than the polar spectrum. This
FIG. 10. DT Tion as measured by individual NIF nTOF detectors minus the
average of all reporting detectors for the T2-only shot (squares) and the
T3He-mix shot (circles), plotted as a function of the detector polar angle.
(The points are artificially separated in polar angle for clarity.)
FIG. 11. In-flight (t¼ 1.75 ns) x-ray image from an equatorial view
(h,u¼ 90�,78�) for T2-only NIF shot N160530-001. The color scale repre-
sents x-ray intensity originating from the SiO2 shell.
041407-9 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
indicates more qR at burn on the equator than on the pole,
and also qR evolution during the time (�200 ps) of nuclear
emission. The difference between polar and equatorial spec-
tra provides further evidence of the poor implosion symme-
try perturbing the measurements. Note, however, that while
evident, the spectral distortions are relatively minor. This
result bodes well for accurate charged-particle spectral meas-
urements at negligible qR in future further optimized NIF
implosions.
Scattered light measurements were also made on these
shots and will help constrain future pointing designs.
Analysis of these data is in progress.
We conclude that while significant progress has been
made towards developing a platform for study of charged-
particle-producing reactions relevant to SN and BBN on the
NIF, some further work is required before it is fully opti-
mized (see Section VII).
FIG. 12. Core x-ray images from the first round of three NIF shots (N160530-001, N160601-001, and N160601-002). The upper row shows the implosions as
viewed from the equator (h,u¼ 90�,78�), while the lower row represents the view from the north pole (h,u¼ 0�,0�). All three implosions behave very simi-
larly. While they look nearly round as viewed from the pole, they are substantially oblate as viewed from the equator, indicating a relatively lower laser drive
in the equatorial plane. (The deviation from round in the top right corner of the polar data is in the same direction as the stalk that holds the target, and is most
likely caused by this engineering feature.)
FIG. 13. D3He-p yields measured in 12 different locations around the NIF
target chamber on T2/3He-mixed-fill shot N160601-002. The red square rep-
resents MRS at h,u¼ 73�,324�, black error bars SRF data, and blue dia-
monds WRF data. The three diagnostic insertion modules (DIM) used to
hold the WRF and SRF detectors are located at h,u¼ 0�,0�, h,u¼ 90�,78�
and h,u¼ 90�,315�, respectively.
FIG. 14. D3He-p spectra as measured on the pole (average for DIM 0�,0�
WRF positions; red), on the equator (average for 90�,78� and 90�,315� WRF
positions; black) and 17� above the equator (MRS, gray). Also shown is the
nominal expected spectrum (dashed curve), assuming Tion¼ 7.1 keV,
YD3Hep¼ 2.7� 108, an instrument resolution of 159 keV and considering
temperature-dependent upshift.28
041407-10 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
VI. CENTER-OF-MASS ENERGY CONSIDERATIONS
An essential aspect of any astrophysically relevant
nuclear experiments is understanding the conditions at which
the reactions are probed. As in a star but unlike in an acceler-
ator experiment, reactions are not probed at a single center-
of-mass (c-m) energy in an ICF implosion. The fuel ions are
expected to follow a Maxwellian (or similar modified62,63)
energy distribution characterized by the plasma Tion.
Because the Coulomb barrier penetrability scales very
strongly with energy, it is primarily the ions in the highly
energetic tail of the distribution that will undergo reactions.
The effective c-m energy distribution of the reacting ions is
described by the Gamow peak.1
In this context, it should be considered that while current
ICF nuclear diagnostics measure a single burn-averaged Tion,
the temperature and density in an ICF implosion are never
homogeneous throughout. The details of the distributions
will vary depending on implosion design. Fig. 15 shows
example Tion and density distributions simulated using the
1D radiation hydrodynamics code HYADES64 for an
OMEGA-scale, 1000-lm OD, 3.0-lm thick SiO2-shell
implosion with 9 atm T2 fill shot with a 30 kJ, 0.6 ns square
laser pulse, and using LILAC for a NIF-scale, 1600-lm OD,
4.0-lm thick SiO2-shell implosion with 5.2 atm T2 fill shot
with a 70 kJ, 1.2 ns ramped laser pulse (burn duration from
this simulation is �70 ps, total qR¼ 10 mg/cm2).
As discussed in Ref. 65, because of the different tempera-
ture dependence of the reactivities for the various reactions of
interest (Fig. 1(b)), variations in Tion and density throughout
the implosion can lead to different burn-weighted Tion for the
different reactions. Using the fact that the yield from any one
fuel element with constant density and temperature and vol-
ume V scales as Y� ninj�hrv(Tion)iij�V, we can calculate
burn-averaged temperatures from the various reactions using
the profiles from Fig. 15 (under the assumption that they are
constant as a function of time). To make a realistic compari-
son to experimental results, we scale the Tion profiles from
Fig. 15 down to match DT Tion measured on the OMEGA and
NIF shots. For the OMEGA profiles (Fig. 15(a)) with Tion
scaled by a factor 0.727, this gives calculated burn-averaged
DT Tion¼ 13.30 keV, TT Tion¼ 13.32 keV, T3He
Tion¼ 13.45 keV, D3He Tion¼ 13.46 keV, and 3He3He
Tion¼ 13.67 keV. For the NIF profiles (Fig. 15(b)) with Tion
scaled by a factor 0.427, this gives calculated burn-averaged
DT Tion¼ 8.22 keV, TT Tion¼ 8.22 keV, T3He Tion¼ 8.67 keV,
D3He Tion¼ 8.66 keV, and 3He3He Tion¼ 9.09 keV. Clearly,
assuming that the simulations accurately describe what is
actually achieved in the experiments, differences in burn-
averaged Tion between these reactions for this type of implo-
sion with relatively flat Tion and density profiles are expected
to be small.
The mean energy and width of the Gamow peak scale as
Tion2/3 and Tion
5/6, respectively.1 Hence, the c-m energy dis-
tribution at which reactions are probed will vary along with
Tion throughout the implosion. A burn-weighted Gamow-
peak can be calculated similarly to the burn-weighted Tion
for different reactions above, by weighting the distribution in
each volume element by the yield expected to be produced
in that volume element. In Fig. 16, Gamow peaks calculated
using the single, burn-weighted Tion for the scaled OMEGA
and NIF simulations from Fig. 15 (TT Tion¼ 8.22 keV and
TT Tion¼ 13.32 keV, respectively) are contrasted to such
burn-weighted Gamow-peaks for the two scaled simulations.
A slight difference is observed between the burn-weighted
Gamow-peak and the Gamow-peak calculated using the
burn-averaged Tion for the NIF simulation with �2� varia-
tion in Tion across the burn region, while virtually no differ-
ence can be seen for the OMEGA simulation with <20%
FIG. 15. Radial temperature (solid black curve) and density (dashed red
curve) profiles calculated using (a) 1D HYADES simulations for an
OMEGA-scale implosion and (b) 1D LILAC simulations for a NIF-scale
implosion (see text for details). The fuel-shell interface is at �128 lm for
the OMEGA-scale implosion and �69 lm for the NIF-scale implosion.
FIG. 16. Gamow-peak energy distributions calculated for the TT reaction
using a single, burn-averaged TT Tion (solid curves) contrasted to effective
burn-weighted Gamow-peak energy distributions calculated using the radial
profiles in Fig. 15 (dashed curves, with Tion scaled down to match Tion mea-
sured on NIF and OMEGA). The left two peaks are for the NIF case (TT
Tion¼ 8.2 keV) and the right two peaks for the OMEGA case (TT
Tion¼ 13.3 keV).
041407-11 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
variation in Tion across the burn region. This result lends con-
fidence that the conditions at which the reactions are probed
are sufficiently well understood.
Since Tion is inferred from the width of fusion product
spectra28 from ICF implosions, other factors than variations
in Tion throughout an implosion may also impact the mea-
sured, burn-weighted Tion.60 As mentioned in Section V
above, an important such factor is directional fuel motion
(flow) during burn, which may lead to additional, non-
thermal broadening of fusion product spectra.58,59 If the
burning fuel assembly is moving in a single direction, this
will lead to a shift in the observed mean energy of fusion
product spectra. Such an effect has been previously observed
for PDD exploding pushers on NIF.66 This effect will not
impact the inferred Tion, and thus not the ability to correctly
infer the conditions at which a reaction is probed. If, on the
other hand, different fuel elements are moving in different
directions during burn (e.g., radial59 or turbulent58 motion),
this will lead to additional, non-thermal peak broadening,
which, if not considered, will result in an over-estimate of
the plasma Tion. Two different scenarios in which this could
happen can be imagined for our exploding pusher-type
implosions: 1D motion, in which case burn happens while
the fuel is moving radially, or 3D motion, in which asymme-
tries in the implosion seed non-uniform flows. For the 1D
case, the inferred Tion would be uniformly inflated around
the implosion, while for the 3D case, variations in the
inferred Tion could be expected around the implosion. The
impact of 3D flow can be studied by measuring Tion in differ-
ent lines of sight. As can be seen in Fig. 10, a line-of-sight
difference of 1.0 6 0.5 keV was observed for NIF shot
N160530-001, while no difference outside of the error bars
was seen for the nearly identical shot N160601-002. At first
glance, these results do not conclusively answer the question
about the impact of 3D flow. The goal for future experiments
is to reduce implosion asymmetry, hence eliminating 3D
flow as a factor.
The impact of 1D radial flow was assessed using post-
shot 1D ARES simulations attempting to match data from
six earlier NIF exploding pusher shots, including N120328.
These simulations indicated flow-enhancement of Tion rang-
ing from 0.2 to 1.7 keV for the six implosions, using a
reaction-weighted average implosion velocity to estimate the
flow broadening58 (we note that this quick estimate gives a
slightly higher value compared to the more accurate treat-
ment of including the impact of flow on neutron spectra from
each fuel element and inferring Tion from the total final spec-
trum). This corresponds to reaction-weighted implosion
velocity variances ranging from 90–310 km/s. Note, how-
ever, that the simulations were struggling to match measured
Tions for these implosions, and that they are inherently lim-
ited in that they are 1D approximations of a highly 2D (or
3D) problem.
VII. DISCUSSION AND FUTURE DIRECTIONS
In this paper, the development of robust HED platforms
for probing astrophysically relevant charged-particle-produc-
ing nuclear reactions on OMEGA and the NIF has been
described. On OMEGA, the platform is fairly mature, with
the first physics results already published11 and more in the
pipeline.12,34 On the NIF, significant progress has been
made, but some further work is required. First and foremost,
the symmetry of NIF implosions must be improved to ensure
reliable charged-particle measurements. Improved symmetry
is also expected to contribute more generally to enhanced
implosion performance. As discussed, achieving symmetry
is challenging because of the NIF beam geometry. Several
avenues are being pursued to learning how to improve laser
drive uniformity for these implosions. First, output from
SAGE simulations is being compared to symmetry results
from earlier implosions, with the goal of finding a way to
adjust the design for improved pointing. For the next round
of NIF implosions, differential beam energies with relatively
higher laser energy in the beams pointed towards the equator
will be implemented in addition to beam repointing. In paral-
lel, 2D simulations using the radiation hydrodynamics codes
ARES, HYDRA,67 and DRACO68 are also being developed
to address pointing optimization. In particular, we conjecture
that cross-beam energy transfer (CBET) as the implosion
converges contributes to the pointing challenges—this will
be addressed using DRACO, which includes a CBET
package.
The first round of implosions on the NIF also produced
lower than desired yields; in particular, the 3He3He-p yield
generated was too low for a high-quality measurement of the3He3He proton spectrum. Comparing the data with results
from reference implosions such as N120328 leads to the con-
clusion that yields were low primarily due to lower-than-
expected Tion for these initial NIF experiments. The most
direct way to obtain higher Tion for these implosions is to
increase the laser energy delivered to the target. Fig. 17
shows the predicted increase in Tion as a function of absorbed
laser energy from a 1D free-fall analysis of 1D ARES simu-
lations of a NIF-scale implosion with OD¼ 1600 lm, shell
thickness 4.5 lm, PT2¼ 5.2 atm and using a 2.1 ns ramped
laser pulse shape. Note that the difference in absorbed laser
energy according to these simulations would need to be
�22 kJ to go from the 11.3 keV Tion observed on N120328 to
the 8.2 keV Tion observed on N160530-001. This does not
seem unreasonable given that the difference in delivered
laser energy between the two shots was �18 kJ (Table II).
However, note that the fraction of laser light that is actually
FIG. 17. Simulated TT Tion as a function of absorbed laser energy from free-
fall analysis of 1D ARES simulations of a NIF-scale implosion with
OD¼ 1600 lm, shell thickness 4.5 lm, P¼ 5.2 atm T2 and using a 2.1 ns
ramped laser pulse shape.
041407-12 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
absorbed by the capsule for one of these implosions (i.e., the
translation factor needed to go from incident to absorbed
laser energy, and hence allow direct comparison between
measurements and simulations) is not well understood at
present.
Longer term, the strategy is to increase the yield of NIF
implosions at a set temperature by going to larger plasma
volumes, thus more fully exploiting the laser energy avail-
able on the NIF. This will allow probing of SN-relevant reac-
tions at more astrophysically germane temperatures. Design
work for this effort is underway. It is expected that the laser
energy would have to increase as capsule radius r3 to achieve
the same plasma conditions with progressively larger tar-
gets.26 If this holds, �16� more yield could be produced
with a 4-mm OD capsule driven with 1.8 MJ than with a 1.6-
mm OD capsule driven with 113 kJ.
A potential challenge that must be handled when infer-
ring astrophysical rates from these relatively low-density
implosions is any kinetic effects in the plasma that may be
impacting implosion performance. Such effects could include:
(i) Tail ion loss, i.e., leakage of the highly energetic ions
from the tail of the ion velocity distribution across the
system boundary due to long mean free paths, as dis-
cussed theoretically in Refs. 62 and 63 and invoked to
partially explain measurements in Refs. 9 and 38.
(ii) Anomalous diffusion leading to separation of the dif-
ferent ion species in a mixed-fill implosion, as dis-
cussed theoretically in Refs. 69 and 70 and invoked to
explain data in Refs. 71 and 72.
(iii) Other multi-ion effects, such as ion thermal decou-
pling, invoked to explain data in Ref. 71.
Most detrimental to the efforts discussed in this paper
would be any undiagnosed kinetic effects that change the rel-
ative yields between different reactions for cases where the
rate of one reaction is to be inferred relative to the well-
known rate of another. This has been shown to be a non-
issue for denser, highly collisional implosions.8 The question
can also be circumvented by taking the ratio of one reaction
branch to another, such as was done for T3He-c relative to
T3He-d in Ref. 11. Kinetic effects will also not impact our
ability to make spectral measurements (although interpreta-
tion of the data could be complicated by kinetic effects, e.g.,
inasmuch as they impact the inference of plasma conditions
such as Tion). For the more general scenario of rate measure-
ments from lower-density plasmas, however, this is a ques-
tion that must be addressed. While this is still work in
progress, a few initial observations can be made based on the
OMEGA and NIF data presented in this paper.
Initial densities for the OMEGA implosions range from
0.8 mg/cm3 for 77 951-77 952 to 2.4 mg/cm3 for 67 941-
68 448. Unfortunately, convergence was not measured for
these implosions. Knudsen numbers (Nk) can be estimated by
using simulated convergence ratios (CRs) from HYADES sim-
ulations of shots 77 951 (CR¼ 4.7), 77 960 (Fig. 15(a),
CR¼ 3.8), and 77 963 (CR¼ 5.6) to be Nk � 5 for the low-
gas-fill implosions (77 951–77 952), Nk � 1 for the high-gas-
fill implosions driven by a 0.6 ns-square laser pulse, and Nk
� 0.1 for the high-gas-fill implosions driven by a 2.0 ns
ramped, de-focused laser pulse (simulated convergence num-
bers are not presently available for the 1.0 ns-square laser pulse
case). Earlier work38 has suggested that the yield drops to
�30% of clean simulated yield for Nk � 0.3 due to kinetic
effects such as tail ion depletion, and that the reduction should
be expected to be exacerbated with increasing Nk.
Interestingly, ARES simulations of the high-gas-fill OMEGA
implosions give an average TT yield-over-clean (YOC) of
�99% for the Nk � 1 implosions driven by a 0.6 ns-square
laser pulse, and YOC � 93% for the implosions driven with
the 1.0 ns-square laser pulse (see Section III). However, this
remarkable result should be taken with a grain of salt because
Tion from the simulations (which were done pre-shot) came in
a bit higher than measured (Fig. 2).
No simulations that reasonably well describe the NIF
implosions exist at present. Available 1D simulations sub-
stantially over-predict Tion for these highly asymmetric
implosions, making any detailed YOC comparisons mean-
ingless. 2D simulations using ARES, DRACO, and HYDRA
are underway which can be expected to better capture implo-
sion performance. Meanwhile, it is interesting as noted in
Section V that performance for the pure T2 and 3He-filled
shots, respectively, seems to scale as expected compared to
the D2-only reference shot N120328 when correcting for
Tion. This is in spite of Nk changing by more than an order of
magnitude from 0.3 for the T2 and D2 shots to 0.01 for the3He-only shot (however, it may be questioned whether a
global Nk calculation is meaningful for these highly asymmet-
ric implosions). On the other hand, performance for the
mixed-fill shot (T2/3He, Nk� 0.05) does not scale as expected
compared to N120328. This might be an indication of multi-
ion effects in the mixed fill. However, note that the T3He-d
and TT-n yield appear to be similarly impacted, which contra-
dicts the hypothesis that anomalous diffusion between T and3He could be responsible for the observation.
VIII. CONCLUSIONS
An inertial-confinement-fusion platform using thin-
glass-shell exploding pusher implosions to probe charged-
particle-producing reactions relevant to SN and BBN has
been developed on OMEGA. Optimized implosions with
OD¼ 1000 lm and shell thickness 3.0 lm driven with a
0.6 ns square laser pulse are demonstrated to produce maxi-
mal yield at the necessary conditions to ensure that undis-
torted charged-particle spectra can be reliably measured, i.e.,
with negligible total qR to minimize ranging in the assem-
bled plasma, and bang time after the end of the laser pulse to
prevent charged-particle energy upshifts due to capsule
charging.
Development of a similar platform is in progress on the
NIF, with the goal of leveraging NIF’s higher capabilities in
terms of laser energy and power to allow probing of astro-
physically important reactions at more stellar-relevant condi-
tions than at OMEGA. Initial NIF experiments struggle with
symmetry due to NIF’s indirect drive beam configuration.
However, charged-particle data from the initial implosions
look promising, and work is underway to address the sym-
metry problems.
041407-13 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
Radial profiles from 1D radiation hydrodynamic simula-
tions are invoked to show that variations in temperature and
density throughout these thin-glass-shell exploding pusher
implosions do not significantly impact our understanding of
the conditions at which the reactions are probed. Future
work includes further exploration of the impact of flows and
kinetic effects in these types of implosions.
Lessons learned from these early proof-of-principle
experiments will be used in studies of other astrophysically
relevant reactions in the future. The long-term goal of this
effort is to explore thermonuclear reaction rates and basic
nuclear physics in stellar-relevant regimes not reachable
through existing terrestrial experimental platforms.
ACKNOWLEDGMENTS
The authors would like to thank the OMEGA and NIF
operations crews for executing these experiments, and Ernie
Doeg, Robert Frankel, and Michelle Valadez for processing of
the CR-39 data used in this work. This work was supported in
part by DOE (DE-NA0001857, DE-NA0002905, DE-FG02-
88ER40387), LBS, NLUF (DE-NA0002035), and LLE
(415935-G).
APPENDIX A: TT YIELD MEASUREMENTS
Measurement of the TT yield presents a challenge for
two reasons: (1) because of the broad energy spectrum of
neutrons from the TT reactions, with the lowest energy neu-
trons not being detectable, and (2) because of previous poor
understanding of instrument sensitivity in this energy range.
As part of the first round of platform development shots on
OMEGA, the TT yield was measured with four different
diagnostics (Fig. 18) using three entirely independent meth-
ods with different systematics:
(a) A bibenzyl-scintillator-based nTOF detector 13.4 m
from TCC, inferring the neutron spectrum from a time-
of-flight measurement.
(b) A magnetic recoil spectrometer (MRS), inferring the
neutron spectrum from measurements of recoil deuter-
ons from elastic n, d scattering in a CD conversion foil
momentum-separated in a magnet to end up in different
physical locations in the detector depending on energy.
(c) Indium and aluminum activation measurements, each
corrected for the contribution from DT neutrons using
copper activation with activation threshold above the
maximum TT neutron energy.
The four diagnostics were found to agree very well. This
addresses point (2) above. Point (1) has so far been addressed
primarily through R-matrix modeling,45 benchmarked by
recent measurements of the spectrum down to �2 MeV.14
This work is still ongoing.
APPENDIX B: FILL CONSIDERATIONS
A significant challenge for the early experiments dis-
cussed in this paper has been to optimize the capsule fills.
Because the reactivity for the DT and D3He reference reac-
tions are orders of magnitude higher than the reactivities for
the T(t,2n)a, T(3He,np)a, and 3He(3He,2p)a reactions (Fig.
1(b)), a primary goal has been to minimize the deuterium
impurity in the fill. The level of deuterium impurity must also
be extremely well known: relative yields between the refer-
ence reaction and the reaction under study can only be used to
infer a reaction rate if the fuel composition is known, and any
uncertainty in the fuel composition will translate directly to
uncertainty in the inferred rate. This problem has not yet been
fully solved for the T2, 3He or T2/3He fuel mixtures with trace
deuterium for either OMEGA or NIF implosions.
SiO2 capsules are filled using a diffusion method, with
the capsules being placed in a manifold which is pressurized
in stages up to the desired capsule fill pressure with time
allowed for the fill to diffuse into the capsule at each stage.
Diffusion rates at room temperature for these types of capsu-
les are of order months to years for hydrogen isotopes, and
of order hours to days for helium isotopes (the higher num-
bers are for capsules flash coated with a �0.1 lm Al barrier
to reduce He leak rates). Hydrogenic fills (D2, T2) are done
at high temperature (300 �C) to increase the diffusion rate of
the capsules for manageable fill times (of order weeks). NIF
capsules have to be flash coated with an Al barrier to prevent
all the helium from leaking out of the capsule during the
�8 h period between opening up a target pressure cell and
imploding the target on NIF. For the NIF T2 fills (expected
D2 impurity �0.1%–0.3%), the level of deuterium impurity
has been evaluated by subjecting residual gas in the manifold
post-fill to mass spectrometry. How representative these
measurements are of what is actually in the capsule is being
evaluated—to date, different experiments have had varying
success in matching measured DT/TT yield ratios to esti-
mated deuterium content. For OMEGA T2 fills (expected D2
impurity �1%–2%), assays of the fuel supply are used to
estimate the deuterium impurity, and they have to be cor-
rected for changes to the composition expected due to impu-
rities being introduced in different stages of the fill cycle.
For 3He fills (expected D impurity �10–100 ppm), a method
with crush tests of a witness capsule for each fill is being
FIG. 18. TT neutron yields measured on the first round of platform develop-
ment shots on OMEGA (Table I) using a bibenzyl nTOF detector (blue dia-
monds), the MRS neutron spectrometer (black solid lines), and indium (red
squares) and aluminum (green triangles) activation, each of the latter two
compared to copper activation to correct for DT neutrons. Note that the
MRS integrated 3–4 shots per measurement, hence the broad lines which
represent an average yield over the covered sets of shots. The dashed black
lines represent the upper and lower uncertainty in the MRS measurement.
041407-14 Gatu Johnson et al. Phys. Plasmas 24, 041407 (2017)
developed to measure the impurity level. (In a crush test, a
capsule filled as if to be shot is instead crushed in a manifold
at high vacuum, and the gas released analyzed using a mass
spectrometer to determine composition.) The crush test
method will also be tried on future T2/3He mixed-fill shots at
NIF.
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